Retrieval and mapping of chlorophyll-a concentration from Sentinel-2 images in an urban river in the semiarid region of Brazil

Abstract

Recently, the Poti river mouth region has experienced environmental impacts that resulted in a change of landscape in its dry season, highlighting the eutrophication and proliferation of phytoplankton, algae, cyanobacteria and aquatic plants. Considering the aspects related to water-quality monitoring in the semiarid region of Brazil from remote sensing, this study aimed to evaluate the performance of Sentinel-2A satellite data in the retrieval of chlorophyll-a concentration in Poti River in Teresina, Piaui, Brazil. The chlorophyll-a concentration retrieval and mapping methodology involved the study of the water surface reflectance in Sentinel-2A images and their correlation with the chlorophyll-a data collected in situ during the years 2016 and 2017. The results generated by the Chl-1, Ha et al. (2017), Chl-2, Page et al. (2018), and Chl-3, Kuhn et al. (2019) equations show the need for calibrating the algorithms used for the Poti River water components. However, the empirical algorithm Chl-2 shows a correlation has been established to identify the spatiotemporal variation of chlorophyll-a concentration along the Poti River broadly and not punctually. The spatial distribution of this pigment in maps derived from Sentinel-2A is consistent with the pattern of occurrence determined by the in situ data. Therefore, the MSI sensor proved to be a tool suitable for the retrieval and monitoring of chlorophyll-a concentration along the Poti River.

Detection and Mapping of Eutrophication and Aquatic Plants in a River in the Northeast Region of Brazil Using Sentinel-2 Data

“…Therefore, the NDCI index obtained the best point performance compared to the MPH index, since the bias values were closer to zero, demonstrating a small systematic overestimation of the chlorophyll-a concentration estimated by Sentinel-2 data and in situ data. According to these results, the relationship between the B04 and B05 bands, is the most suitable for the recovery of chlorophyll-a in the Poti River, which is in conjunction with observations made by Pereira et al (2020). NDCI values obtained from the Sentinel-2 showed good results in the Tietê River Basin (Lobo et al, 2021).…”

“…The study area corresponds to the 36.8 km urban stretch along the Poti River, located in Teresina, Piaui, Brazil. As reported by Pereira et al (2020); the Environmental Sanitation Laboratory, Federal University of Piaui, carried out monthly sampling of chlorophyll-a concentrations at seven monitoring points located on the Poti riverbed, in 2016 and 2017. The collection points are in easily accessible places, as well as in regions with little urban intervention or more urbanized on the Poti River in Teresina.…”

“…To explore the concentration of chlorophyll and the proliferation and aquatic plants, the following data sets were formed, respectively: i) eight products level L1C, considering the dates of Pereira et al (2020); ii) six L2A level products, with the following acquisition dates: September 14, 2019 and October 14, 2019, from Sentinel-2B; October 29, 2019, November 8, 2019, December 8, 2019, and December 28, 2019, of Sentinel-2A.…”

“…In situ data of chlorophyll-a, total phosphorus and determination of trophic status index It is noteworthy that in this study the same 56 samples of chlorophyll-a are used in situ and the same eight L1C products from the Sentinel-2A satellite, arranged in Pereira et al (2020). Therefore, there are the same number of match-ups that varied with a difference of one to twelve days, in all the period available in the dry season.…”

“…The process of accelerated and disordered growth in the city of Teresina, located in the Northeast region of Brazil, has caused environmental problems that affect the Poti River, such as eutrophication and the appearance of proliferation of phytoplankton, algae, cyanobacteria and aquatic plants that have occurred, notably, since 2003, in the dry period of the year (Brazil, 2007;Costa, 2014;Santos, 2017). In a recent study, researchers from the Environmental Sanitation Laboratory of the Federal University of Piaui (UFPI), estimated the concentration of chlorophyll-a in seven points located on the Poti riverbed, during the proliferations that occurred in the years 2016 and 2017 (Soares et al 2019;Pereira et al, 2020). However, in 2019, there was a great proliferation of aquatic plants, especially water hyacinths or Eichhornia crassipes (Bom Dia Piaui, 2019).…”